Photocathodes have been used in opto-electronic devices, such as TV camera tubes, image tubes, motion detectors and counters, etc. High quantum efficiency (QE) and long-life characteristic have been desired for photocathodes. Currently available photocathodes may only last for a matter of hours in the vacuum environment of an electron gun. Their emission efficiency degrades over time in a practical vacuum environment because of trace amount of gases, which contaminates and degrades the sensitive photocathode film. One of the principle challenges for photo-injection is extending the lifetime of high efficiency photocathode operation.
Further, while graphene layers have been contemplated as a protective layer for photocathodes, a macroscopic substrate typically included in conventional photocathodes makes it difficult to exploit features that can only be realistically achieved in free space suspension.